The present invention relates to methods and apparatus for the application of protective film lamination. In particular, the invention relates to the lamination of documents for their protection and preservation. Machines and methods for applying a protective film lamination to documents is well established and represents a substantial industry. The present invention relates particularly to laminating methods and machines that employ plastic laminating material packaged in a roll, a laminating plastic material supply roller on which the plastic material is mounted, a guide member having a surface over which laminating plastic material is passed in contact therewith, laminating rollers through which laminating plastic material is guided by the guide surface and where pressure is applied, and drive means for causing laminating plastic material to be fed off of the supply roller, over the guide surface and through the laminating rollers. An example of such a machine is the 6000 Series laminator manufactured by GBC, Inc. of North Brook, Ill., U.S.A. which is of the type adapted to use heat activated laminating films such as those offered by Graphic Laminating, Inc. of Cleveland, Ohio.
In machines of this type the guide member is part of a heat shoe which heats the plastic material to the temperature necessary to activate the adhesive qualities of the film. When pressure sensitive films are used the adhesive quality of the film is made available by removal of a release liner in which case the guide surface may or may not be heated. The invention will be described with reference to the type of machine in which the guide surface is heated although the invention is adaptable equally to machines in which the guide surfaces need not or is not heated.
The laminating machines to which the present invention relates perform their laminating function by feeding laminating plastic films from two separate supply rolls over individual heated guide surfaces called "heat shoes" where the adhesive qualities of the films are activated. With the films heated to the required temperature, the document to be laminated is inserted between the films after which all three components are passed together through the laminating rollers which apply pressure to the laminate thereby sealing the document (substrate) to the protective layers of laminating film.
A properly laminated document is one that has a clear, wrinkle- and bubble-free coating of plastic film adhered to it which does not curl after the heated plastic cools. In order to achieve these desired results, it is imperative that the film applied to the document be wrinkle-free and not stretched to the extent that will cause shrinkage when the film cools. While these results are being realized by machines currently available in the market and known in the prior art, they are doing so using methods and apparatus which have inherent deficiencies that cause undue wear and tear on machine components, require constant operator attention, result in substantial film waste, limit the operating speed of the process and require film manufactured to high tolerances. Other disadvantages found in prior art machines will be discussed below.
In the typical laminating machine of the type to which the present invention relates, the driving force that draws the laminating plastic film material over a guide surface (heat shoe) and through the laminating rollers is provided by drive means usually in the form of a pair of drive rollers which frictionally engage the film and move it through the system. It is essential for heat activated film that the entire surface of the film be heated before it contacts the substrate and prior to the film being led between the laminating rollers in order that the adhesive of the film be activated over its entire surface. Any wrinkles in the film as it passes over the heat shoe represent areas of the film not contacting the heat shoe surface and thus likely not heated sufficiently to activate the adhesive qualities of the film. In addition to poor adhesion, any wrinkles that are permitted to exist when the film is drawn between the laminating rollers will result in an uneven and unacceptable surface on the document being laminated. Thus, it is essential for both heat activated as well as pressure sensitive films that all wrinkles be eliminated before the films are brought into contact and enter between the laminating rollers.
In order to eliminate wrinkles in the laminating film material as it passes over the guide surface, prior art machines impose a substantial drag force on the rollers that carry the supply roll of plastic laminating film. The drive rollers that move the plastic film through the system provide the force to overcome the drag on the supply rollers, which force is transmitted through the film itself. This force in the film stretches the film to eliminate wrinkles at the guide surface. When the drag force of a supply roller is properly adjusted it will turn only when the tension on the film itself as applied by the drive rollers is sufficient to eliminate wrinkles in the laminating material.
Because the diameter of the plastic laminating material roll decreases as the plastic is used, an increasing amount of tension in the film is required to overcome the drag force on the supply rollers. If adjustment is not made to diminish the amount of drag force on the supply rollers as the diameter of the roll diminishes, the tension in the film can build to the point where it causes distortion in the film which in turn results in curl in the laminated document or ultimately breaks. Thus, it is necessary with prior art devices to change the drag force applied to the supply rollers as material is consumed from the roll. Most of the laminators presently in use and known in the art require that the drag force on the supply rollers be frequently manually adjusted requiring constant attention during the laminating process and the skill of an experienced operator. Of late, some machines have been equipped with means for automatic adjustment of the drag force on the supply rollers through the use of sensors and microprocessors, but these devices have proven to be less than reliable and greatly add to the cost of the machines.
One of the problems attendent to using supply roller drag force as a means of wrinkle elimination is that of film breakage on start-up. In order to start the film moving it is necessary to overcome the drag forces on the supply rollers. Since the start-up force to get the rollers moving is substantially greater than the force required to maintain the rollers moving, film is likely to break if the drag force on the supply rollers is not reduced prior to start-up. If start-up is attempted after the heat shoes have been activated and are up to temperature, the heated film is even more likely to break. Thus, either the heat shoes have to be cooled down which is very time-consuming, or the drag force on the supply roller must be greatly reduced and then readjusted as the film begins to feed between the drive rollers. In either event, the start-up is time-consuming and requires a substantial amount of film to be fed through the drive rollers before actual document lamination can begin. The waste of material is not insubstantial.
The fundamental problem with prior art devices can be found in the fact that it is not possible to create a uniform tension across the width of the film at the location of the guide surface by simply imposing a drag force on the remote supply rollers. In fact, the wider the film the more difficult it becomes to eliminate wrinkles in this way and the greater the forces required. The inability to establish a uniform tension across the film on the guide surface derives from several sources including non-uniformities in the film itself, uneven pressure applied by idler bars between the supply rollers and guide surface, uneveness in the guide surface, unequal forces applied by the devices that apply the drag force as well as different rates of wear in the various machines bearings. Where the irregularity in the film causes forces in the film to be too unevenly distributed the material must be rejected since it is not possible to eliminate wrinkles without breaking or seriously distorting such film.
In order to eliminate all the wrinkles using the prior art method, the drag force must be sufficient to create tension in the area of the film carrying the least amount of force, but this creates more force than necessary in the rest of the film. This greatly burdens the machine parts themselves and often causes distortion in the final product. Because of these inherent difficulties, laminating films more than thirty inches wide pose serious problems making laminating very difficult, and only possible with machines capable of handling substantial forces.
As the speed of the laminating process increases, even greater drag forces on the supply rollers is demanded in order to eliminate wrinkles. Because the films and machines can only carry so much force and continue to operate, speeds must be maintained low enough to avoid the application of destructive forces. Thus, the wider the film, the slower the process.
The methods and apparatus of the present invention assure a constant supply of wrinkle-free heated laminating plastic material to the laminating rollers by a means totally independent of the force required to turn the supply rollers. By eliminating the substantial drag forces on the supply roller and by not imposing any force of like magnitude on the plastic film anywhere else throughout the system, the film is fed through the system unstretched and with forces well within the tensile strengths of the film itself even when it has been heated to the temperature required to activate its adhesive qualities. Accordingly, the present invention permits immediate film start-up with the heat shoes at operating temperature without resultant film breakage or stretching thus eliminating machine down time and wasted material. In addition, the present invention permits operation at greatly increased speeds over those attainable where the traditional supply-roller drag forces are used as the means for eliminating wrinkles. One of the remarkable advantages of the present invention is that the methods and apparatus thereof are fully adaptable to existing machines requiring only a relatively slight modification thereto. Thus, the present invention permits substantial upgrading of the productivity of existing laminating machines without the capital investment necessary to replace the machine altogether. In addition, the forces on machine parts are greatly reduced significantly adding to the expected useful life of the machine.